Babies who gain weight faster than normal during their first six months of life are more likely to be overweight at age 2, which could foreshadow obesity during childhood and beyond, British and Canadian researchers report.

Doctors know from earlier research that babies born small are more likely to be obese later in life and that children who are overweight at age 2 are more likely to be obese as adults.

This study of 1,650 babies focused on the rate of weight gain in their first six months and concluded that the track to obesity is set by the half-year mark.

"This is a new marker that might be a predictor of future body weight," says Andrea Dunaif of Northwestern University in Chicago and president of The Endocrine Society. "Obesity is incredibly hard to treat. Ways to identify children at risk are very important, so we can focus prevention efforts on those children."

Surprisingly, what and how much babies were fed had no effect on the findings, the researchers noted. Their report was presented Saturday to The Endocrine Society, whose meeting ends today in San Diego.

One possibility for the marker is that during pregnancy, a metabolic switch is activated in the fetus, causing the baby to be born small but with the ability to store extra calories efficiently as a survival mechanism, says Kenneth Copeland, University of Oklahoma professor of pediatric diabetes and endocrinology.

The researchers did not report a cause-and-effect relationship in their findings. "If a baby gains weight real rapidly, it predicts obesity in childhood. That doesn't necessarily mean it causes it," Copeland says.

The next step is more research to see whether the relationship between rapid weight gain in infancy and later obesity can be influenced by breast-feeding, nutritional improvements or other interventions, he says.

Even though what the babies were fed was not a factor in the study, Copeland believes parents should avoid overfeeding children.

"We still have a society where a large number of families believe a fat baby is a healthy baby," he says.

But Dunaif cautions against putting infants on diets. "Kids need a certain amount of fat intake, and they need milk. This study shouldn't provoke parents to, on their own, start modifying (the infant's) diet."